Sharing users,sessions,profiles across multiple servers and DB's? (or building a massively scalable feature driven architecture)

Yesterday my site that averages 200/2000 authenticated/anon users during peak hours topple over due to some serious MySQL locking. We're run a reasonably complex site relying on CCK, Buddylist, OG, Private Message and lots of Views.

Wherever we turned, we get the same answer "tune your MySQL queries". While there is so only so much tuning one can do, it also becomes problematic when your site is growing and you want to roll out new features.

We've been brainstorming a solution and came up with an architecture that would sounded like it would solve our problems (in theory).

We would use one server to host the main site (www.example.com) with core functionality (forums, events, blogs) + aggregated feeds of a number of other subsites (each dedicated to a single feature). These would all be on a second server where each feature would run in a seperate VPS instance with it's own database and httpd. It would look something like this:

example.com - main server aggregating feeds from:
|----> groups.example.com second server - VPS1
|----> classifieds.qatarliving.com second server - VPS2
|----> newfeature.qatarliving.com second server - VPS3
etc.

Our thinking is that if any of these features grows we can just move the VPS onto a new server. Also, if any of these features spike, we can take them down without affecting other services.

To the user it should be seemless since we will aggregate all the data to the main site through RSS feeds.

One of the key requirements would be the sharing of the user table, sessions, userpoints and profiles across all sites (i.e. all the subsites on server two should use those tables from the DB on server 1). Users should be able to move between sites without having to login again and if they get points for posting on a subsite it should increment the main site.

My questions are:
1. Does this make sense at all? Or is there a much simpler and more obvious way to do this?
2. Is it possible to share those tables on a DB that is on a seperate machine altogether?
3. Any other advice?